1. Field of the Invention
This invention relates to an apparatus and method for securing a flexible line on an elongated structure and, more particularly, relates to securing a flexible line, such as a flexible flow-line or an electromechanical cable, in a pre-existing channel region on a support member of an offshore structure.
2. Description of the Prior Art
Within the last few years offshore petroleum activity has been increasing at a significant rate worldwide. This in turn has prompted the development of more sophisticated offshore structures for the exploration and production of oil and gas. In addition, the world demand for oil and gas has necessitated improvements in the manner of installing facilities for the transportation of oil and gas from the offshore structure to an onshore refining facility. Traditionally, oil and gas are transported by means of an underwater pipeline running from the offshore structure to the onshore facility or, alternatively, from the offshore structure to an adjacent offshore storage facility adapted to accommodate ocean-going tankers. The underwater pipeline is attached in its vertical attitude to a support member of an offshore structure, e.g., the leg of a platform or the tension member of a mooring buoy system such as that disclosed by Hauber, U.S. Pat. No. 3,557,564, and Broadway et al, U.S. Pat. No. 3,503,218. The flow lines which are attached to structures located in deeper water are preferably flexible since these structures, due to their generally slender geometry, undergo large lateral deflections in response to wave loadings and related environmental forces. In addition, mooring buoy systems such as that disclosed by Flory et al, U.S. Pat. No. 3,641,602, and Flory et al U.S. Pat. No. 3,614,869 often include a swivel assembly along the length of the tension member. This requires a flow line resilient enough to pass around a universal joint of the tension member permitting pivotal movement of the joint without damaging the flow line or interferring with the flow characteristics of the line.
Besides flow lines, offshore platforms frequently obtain their electrical power from onshore rather than generating it with equipment on-board the production platform. The power can be transmitted via a cable directly from the shoreline, or, if the production platform is tied to an offshore storage facility, the power generating equipment can be mounted on the offshore storage facility thereby transmitting electrical power directly from the offshore storage facility to the platfrom. However, the same problems associated with flexible flow lines occur with respect to electrical cables. For example, if the electrical generating power equipment is housed onboard an offshore storage facility which is moored to the sea floor by a single leg riser system, the cable must be sufficiently flexible to pass around the universal joint of the tension member and, subsequently, attach to the leg of the offshore platform without damaging the inner core of the cable.
Frequently, the electrical cable or flow line is installed on an offshore platform via a J-tube assembly as disclosed by Otteman et al, U.S. Pat. No. 3,434,296, or via a riser assembly as disclosed by Matthews, U.S. Pat. No. 3,702,539.
With respect to a single anchor leg mooring (SALM) 16 system as illustrated in FIG. 1, the preferred practice is to suspend an electrical cable 15 from the top end of a support member 26 near the water surface to a second submerged buoy 18 at a predetermined distance from the SALM (i.e., 1000-2000 yards). The second buoy supports the cable at a predetermined depth approximately midspan between the support member and the sea floor. The buoy in FIG. 1 is restrained by a set of anchor cables 19. In this manner, unloading tankers may berth adjacent the SALM without interferring with the second buoy support. In deeper water, the second buoy 18 is restrained a predetermined depth below the water surface by the submerged weight of the cable 15 as illustrated in FIG. 2.
Occasionally, the flow lines of a SALM unit are not suspended as illustrated in FIGS. 1 and 2 but, rather, are attached as illustrated in Flory et al, patents '602 and '869, to a tension member. The flow line may be an integral part of the tension member of the SALM, i.e. passing within the tension member, or attached by means of clamps, such as that illustrated in Hauber, to the exterior surface of the tension member.
However, several problems have been encountered with the conventional methods for securing electromechanical cables and flow lines on both fixed offshore production platforms and SALM systems. The J-tube assembly requires a vertical hook load to pull the cable or line through the assembly due to the frictional forces between the line and J-tube. If the hook load is lost, the cable will slip back into the J-tube coiling itself within the tube. The second buoy system, illustrated in FIG. 2, is particularly vulnerable to environmental load problems. The wave and current loads on a line suspended over several thousand feet are substantial. Typically, the line is not laterally restrained; but rather, the line is permitted to wander freely with respect to the SALM terminal. This poses a problem for tankers attempting to approach the SALM terminal and dock. Therefore, industry has suggested the need for an improved method and apparatus to install and maintain flexible flow line and electromechanical cables adjacent fixed offshore platforms and SALM terminals.
Additional patents of general interest are:
U.S. Pat. No. 2,119,996--Massholder
U.S. Pat. No. 3,572,408--Hnot
U.S. Pat. No. 3,595,312--Matthews, Jr.
U.S. Pat. No. 3,606,397--Flory
U.S. Pat. No. 3,708,811--Flory